Adding to what has been said before on Unlimited Express, they're sold the day of the event. And it certainly is a limited amount, for when I did it last year, I got up early and went to the vacation center desk at Stella Nova and they only had a handful of them left at that point.
Now something I also learned was asking someone manning the Express ticket kiosks inside the Studios. The Unlimited Express tickets go on sale at midnight, so if you're able to be at HHN the night before, you can totally get the upgrade at Midnight. Just make sure you have your single night express ticket and you're good to go.